Cardiomyocytes in the adult mammal exhibit little if any capacity to undergo cell division. The ability to reactivate cardiomyocyte proliferation in a diseased heart could be of considerable therapeutic value if the newly replicated cells are able to contribute to contractile function. We have recently generated transgenic mice that express D-type cyclins in the heart, in order to determine the effects of forced G1/S transit on cardiomyocyte cell cycle activity. Expression of cyclin D1, D2, or D3 promoted low levels of cardiomyocyte DNA synthesis in adult transgenic hearts. Surprisingly, myocardial infarct and beta-adrenergic stimulation markedly increased the rates of cardiomyocyte DNA synthesis in peri-infarct zone of the ventricle and left atrium, respectively, in transgenic mice that express cyclin D2. These features appear to be specific for cyclin D2, as both cardiac injury and beta adenergic stimulation markedly reduced cardiomyocyte DNA synthesis in transgenic mice expressing cyclin D1 or cyclin D3. Subsequent studies have shown that DNA synthesis can culminate with cardiomyocyte cytokinesis in the cyclin D2 model. Our results indicate a fundamental difference in the capacity of the D-type cyclins to promote cell cycle activity in cardiomyocytes, and also suggest that cyclin D2 might be used to affect myocardial regeneration following injury. We propose four Specific Aims to study the effects of cyclin D2 expression in cardiomyocytes. Aim 1 will characterize the response of ventricular cardiomyocytes to cardiac injury in MHCcycD2 transgenic mice; these experiments will determine the extent to which cell cycle activation can ameliorate muscle loss following myocardial infarction, as well as test the capacity of cyclin D2 to induce cell cycle activity in naive adult ventricular cardiomyocytes. Aim 2 will establish the molecular basis for the differential effects of D-type cyclins in cardiomyocytes; these experiments will test the hypothesis that differential nuclear-to-cytoplasmic trafficking underlies the differential capacity of the D-type cyclins to promote cell cycle activity in response to myocardial injury. Aim 3 will characterize the response of left atrial cardiomyocytes to beta-adrenergic stimulation in MHC-cycD2 transgenic mice; these experiments will establish the degree to which cyclin D2- mediated adult cardiomyocyte cell cycle activity can be driven, as well as characterize the functional activity of replicated atrial cells. The proposed experiments will further delineate the consequences of cyclin D2 expression in the adult heart, and should provide additional information regarding the regulation of the cardiomyocyte cell cycle. Ultimately, these experiments will determine if manipulation of the cyclin pathway can be exploited for the induction of therapeutic myocardial growth in the adult heart.